In prior art structures, an example of which is represented by an accelerometer seismic assembly of the type disclosed in Jacobs, U.S. Pat. No. 3,702,073 assigned to the assignee of this application, the transducing assembly, in this case a seismic assembly, is supported on a flexure connected to a support ring outer member supported between the upper and lower stator assemblies of the accelerometer. The seismic element, in this particular example, includes a force restoring coil and an arcuate pick-off capacitor plate, and is connected by means of one or more flexures to the annular outer support ring. In this instrument the seismic assembly including the support ring and flexures is configured out of a unitary piece of fused quartz.
One of the objectives in designing instrument assemblies and their associated support structures, such as shown in U.S. Pat. No. 3,702,073, is to minimize the effect of stress on the flexure elements connecting the moving element or mass to the outer support structure. Strain in the flexures supporting the seismic mass resulting from stresses in the support structure can result in significant bias errors. In the example of U.S. Pat. No. 3,702,073 which discloses a servoed accelerometer seismic assembly, the pick-off elements are used to produce signals indicating the position of the assembly within instrument which in turn are used to generate a current in a restoring coil attached to the seismic assembly so as to restore the seismic mass to a predetermined position within the instrument. Stress in the flexures in the servoed position can result in an undesired bias in the output signal since the instrument will be attempting to overcome any forces generated in the flexure. In the case of an open-loop instrument the stress in the flexure is free to cause a motion of the pick-off, also producing a bias error in the output. One source of stress forces that can be transmitted to the flexures results from the method for securing support member to the stator elements of the assembly. Since the surface of the stator elements abutting the annular support ring cannot as a practical matter be made perfectly flat, the securing of the support ring, which is also not perfectly flat, to the stator element will in most cases produce stress forces in both the outer support member and the flexures.
One approach to reducing the stress transmitted to flexures in a transducer assembly is disclosed in U.S. Pat. No. 3,702,073. The annular outer support member is clamped between stator elements where spacer elements or pads are interposed between the faces of the stator members and the support ring. In this approach three pairs of pads are spaced approximately 120.degree. around the support member. However, even with this approach undesired stress forces can be generated in the annular support ring which are transmitted to the seismic mass support flexures.
A second approach to reducing the effect of mounting stress is shown in Wilcox U.S. Pat. No. 3,339,419. The Wilcox patent discloses a design wherein the support member is cantilevered, in turn supporting a cantilever mounted movable element with essentially one half of a circular movable element and circular pick-off means on each side of the cantilever support edge. This approach reduces sensitivity to strain rotation by having essentially equal areas of the movable element and a circular pick-off means on each side of the line of mounting of the support member.
It is not, however, usually either practical or even desirable to use a circular pick-off area. It is indeed more efficient to concentrate the pick-off area at a maximum radius from the pivot axis in a pendulous transducer, to maximize pick-off angular gain.